System for the clarification of sewage and other liquid-containing wastes

ABSTRACT

A novel sewage purification system is provided herein. The system includes a pump for drawing sewage into the system and an injection inlet for injecting chemical into the effluent line from the pump. The pump leads to a pair of interconnected U-shaped mixing chambers connected to the effluent line from the pump, the U-shaped mixing chambers being baffled and being of a particular construction. There is also a recycle line from the inlet line to the mixing chambers back to the pump for recycling about 1-20% of the total flow rate, which is generally about 5-40 gallons/minute through the system. The U-shaped mixing chambers are also provided with at least two injection inlets for injecting chemicals thereinto. A splitter chamber is connected to the outlet from the mixing chambers to provide a pair of parallel flow outlets therefrom. A separator chamber is connected to the parallel flow outlets, the separator chamber having structure therein for providing tangential flow along the inside walls and also for separating sludge therefrom and for withdrawing it from the bottom thereof, and for withdrawing clean water from the upper central core thereof. Finally, a sludge removal line is provided, along with a sludge recycle line for recycling about 1-20% of the sludge back to the pump inlet line.

BACKGROUND OF THE INVENTION

(i) Field of the Invention

The present invention relates to water treatment, and more particularlyto an improved water treatment system used to produce drinkable waterfrom impure water.

(ii) Description of the Prior Art

Conventionally, sewage is first pretreated in lagoons to settle outsolids and it then passes through first and second filtering tanks orthe like which attempt to remove remaining solids. The effluent is thenallowed to pass to the disposal outlets which may be rivers, streams orthe like. Unfortunately, this effluent still contains a relatively largequantity of impurities, the method is time consuming and relativelylarge land areas are required for the lagoons.

A so-called "package water treatment plant" is appropriate for use insmall community water systems, resort areas or construction and lumbercamps located in remote areas. Such package water treatment plant whichis easily maintained and operated by unskilled individuals is preferableover larger more complex systems requiring continuous maintenance andsupervision. In particular, there is a definite need for packagetypewater treatment plants which can accommodate flows from about 300 to6000 gallons per hour and can operate on a continuous daily basis. Thesize of a package water treatment plant is important, and it isdesirable to provide a plant which may be readily moved from one area toanother depending upon needs.

It is also well known in the art that the addition of certain chemicals,such as lime and soda ash, to water having undesirable solids dissolvedtherein has the effect of causing the dissolved solids to precipitateand form a suspension or slurry with the water. It is further known thatto add raw water and chemicals to a slurry formed as above results inimproved and accelerated coagulation and purification of the water. Thesolids which precipitate from the raw water tend to deposit by accretionon the particles present in the slurry, forming relatively stableparticles which are readily separable from the clear, purified water.

It is also well-known that color, turbidity, organic matrer and similarimpurities may be removed from water by coagulants, e.g. alum, ferricsulfate or the like. These compounds are acidic and react with thealkalinity in the water or with alkaline compounds, e.g. lime or sodaash, to form voluminous insoluble precipitates (hydrates). Theprecipitates have a tremendous surface area on which the dissolved orcolloidally dispersed impurities are absorbed. The suspended impuritiesare surrounded by the gelatinous hydrates and become part of theprecipitate.

To soften water by this process, lime (calcium hydroxide) is added toprecipitate the calcium bicarbonate as calcium carbonate and themagnesium salts as magnesium hydroxide. Soda ash (sodium carbonate) isadded to react with the calcium chloride and calcium sulfate originallypresent in the water as well as that formed by the reaction of lime withmagnesium chloride and sulfate. The reaction of sodium carbonate withthese salts forms calcium carbonate. Thus the hardness (calcium andmagnesium salts) originally present in the water is partially removed asthe slightly soluble compounds, calcium carbonate and magnesiumhydroxide, precipitate.

Usually a coagulant, e.g. alum, sodium aluminate or ferric sulfate, isadded in the treatment to assist in the separation of the turbidity andprecipitates formed from the water. If the sterilization and reductionin organic mater are required, chlorine is also used in the treatment.By suitable modification in the chemical treatment, silica reduction canbe obtained.

Domestic or sanitary sewage and industrial wastes may be purified by thechemical precipitation process, in which suitable chemicals (e.g.aluminum sulfate, lime, iron chloride, polyelectrolytes or combinationsthereof) are added to the sewage and the sewage passed to one or moreflocculating tanks, normally equipped with slowly rotating agitators orpaddles, in which colloidal solids are formed into particles of size andweight that will settle. The colloidal solids or flocs are thenseparated from the liquid by being allowed to settle in subsequentsettling tanks, whereafter the purified water is collected in a weirstructure mounted at the surface of the water, while the sediment,consisting of flocs and sludge, is removed, normally by means of sludgescrapers and/or pumps. Certain industrial wastes or sewage has inherentflocculating tendencies, and thus it is unnecessary with such effluentsto add flocculating chemicals since corresponding purifying effects canbe obtained solely by agitating the liquid and allowing the resultingfloc to settle.

Petersen, Canadian Pat. No. 517,674 issued Oct. 18, 1955 provides methodand apparatus for the purification of water. The method for treatingwater to remove substances therefrom by chemical precipitation comprisescontacting the water to be treated with partially clarified water andforming a mixture thereof. Chemicals are added to the mixture so formedand the mixture is passed into a reaction zone under sufficient pressureto cause agitation therein. A sufficient portion of the water displacedfrom the reaction zone into a primary clarification zone to give a flowrate of from 2 to 14 gallons per minute per square foot ofcross-sectional area in the primary clarification zone, the incoming rawwater is contacted therewith. The remaining water is displaced from thereaction zone into a secondary clarification zone wherein theprecipitated solids settle out slowly. Treated, clarified water is thenwithdrawn from the secondary clarification zone.

The water treating apparatus includes a treating tank, an inner shellextending downwardly within the tank to a point near the base thereofand communicating at its bottom with the interior of the tank. The innershell and the tank form an annular passage therebetween. The inner shellhas an open-bottomed base portion of greater cross-sectional area thanthe upper portion thereof. Communicating means are provided between thebase portion and the upper portion. A raw water inlet is provided in theupper portion of the inner shell. Means are so arranged as to withdrawwater from the upper portion of the inner shell and to introduce suchwater into the base of the tank at a point immediately below the openbottom of the base portion of the inner shell. Means are provided forintroducing chemicals into such water after withdrawal from the innershell and prior to introduction into the tank. Agitation stilling meansare mounted in the annular space between the base portion of the innershell and the tank. Means are provided for removing solids precipitatedfrom the treated water. Finally, means are provided for removingclarified, treated water from the upper portion of the annular spacebetween the inner shell and the tank.

A package water treatment plant is disclosed in Canadian Pat. No.692,543 issued Aug. 11, 1964 to C. L. Oldfather. The water treatmentplant includes a cylindrical shell disposed with a semicircular settlingtank. The cylindrical shell incorporates a plurality of coagulationchambers arranged in continuous communication for eventual discharge ofinfluent impure water into the settling tank. The settling tankdischarges water therefrom into a filter which uses sand as thefiltering media. After passing through the sand filter, the water isthen transported to a storage tank.

Duff et al, Canadian Pat. No. 843,762 issued June 9, 1970 provides awater treating apparatus for softening of water by the cold lime-sodaash process and for the clarification of waters containing suspendedsolids, color and organic impurities by coagulation with alum or ferricsulfate or other coagulants. Coagulation and softening may be carriedout simultaneously in this type of water treating apparatus. Such watertreating apparatus has means forming a settling zone, means forming amixing and recirculation zone and means forming an uptake zone. Theuptake zone has a closed bottom member with an opening substantially inthe center thereof through which the uptake zone communicates with themixing and recirculation zone and the settling zone, and the upperportion of the intake zone communicates with the mixing andrecirculation zone. The mixing and recirculation zone communicates withthe settling zone. Means are provided to supply a chemical treatingagent to the uptake zone to form precipitates therein. Means areprovided to withdraw effluent from the settling zone. Feed means areprovided to pass influent water into the uptake zone, those feed meansto pass influent water into the uptake zone being located above thebottom member. These feed means are so constructed and arranged and arethe sole means for causing the water in the uptake zone to rotatetherein and to be pumped upwardly from the uptake zone and forrecirculating precipitates entering the uptake zone from the settlingzone through the opening in the bottom member thereof. Those feed meansto pass influent water into the uptake zone are connected to a supply ofinfluent water.

Miller, et al Canadian Pat. No. 853,022 issued Oct. 6, 1970 provides ahot process settling tank which includes a generally spherically shell.A partition divided the volume of the spherical shell into an upperreaction compartment and a lower settling compartment. An upwardlydiverging baffle is disposed within the spherical shell forming the basesurface of the settling compartment and defines the upflow zone ofincreasing cross-sectional area for gradually decreasing the rate ofupflow to permit the formation of sludge blanket in the settlingcompartment. The baffle and spherical shell together form a generallyannular space therebetween which may be used as a liquid storage. A ventis associated with the liquid storage to prevent the entrapment of airtherein as the storage space fills with liquid.

Dunhers, Canadian Pat. No. 972,880 issued Aug. 12, 1975 provided asewage flocculating and sedimentation tank unit for purifying domesticsewage and industrial waste, comprising a flocculating section and asedimentation section having an outlet and a fluid connectiontherebetween. The flocculating section and sedimentation section and theconnection therebetween are positioned so that the flow of sewage fromthe flocculating section to the sedimentation section outlet, as seen inplan view, is directed substantially perpendicular to the longitudinaldirection of the tank and as seen in vertical section, obliquelyupwards. Each of the flocculating section and the sedimentation sectionis divided into a plurality of aligned compartments by transverselyextending guide walls for substantially uniformly distributing thesewage over the tank in the longitudinal direction thereof. Thoseplurality of aligned flocculating compartments extend the longitudinallength of the tank. The sedimentation compartments confront oppositesides of the flocculating section in fluid communication therewith.

Bultz, Canadian Pat. No. 951,035 issued July 9, 1974, provides a sewageand effluent treating system. The secondary sewage treatment apparatusincludes means to add a flocculating chemical to the sewage. Means areprovided to agitate the sewage and the chemical thoroughly to mix them.Means are provided to separate the flocculated material from the liquideffluent. Such means to agitate the sewage and the chemical includes anagitating module. The agitating module includes a casing, an intakeheader at one end of the casing, an outlet baffle chamber at the otherend of the casing and a plurality of conduits in the casingcommunicating between the header and the baffle chamber. Each of theconduits include a plurality of baffles extending from their walls toagitate sewage passing therethrough. A substantially horizontalperforated partition baffle is provided which spans each of the conduitslengthwise and separates each conduit into a normal flow agitatingportion above the partition baffle and a reverse flow cleaning portionbelow the partition baffle. The plurality of baffles are located in thenormal flow agitating portion. An effluent-flow-operated flap valve ishinged on the discharge ends of each of the conduits for closing off thenormal flow agitating portions when effluent is flowing in a directionopposite to the normal direction of flow, and for opening the normalflow agitating portion when the effluent is flowing in the normaldirection of flow.

In U.S. Pat. No. 3,768,648, is disclosed a settler assembly whichutilizes a plurality of inclined passages of chevron cross-sectionconfiguration. The chevron configuration provides a higher critical flowrate than passages having square, circular, hexagonal and the likecross-sectional configuration.

Swelberg, Canadian Pat. No. 1,074,928 issued Apr. 1, 1980 provides apackage water treatment plant for producing drinkagle water from impurewater. It incorporates a plurality of coagulation chambers incommunication with a primary settling tank which in turn is incommunication with a secondary settling tank. The secondary settlingtank incorporates a plurality of inclined coextensively arranged chevroncross-sectional settling tubes which greatly increase the rate ofsedimentation of the impure water for subsequent delivery to a thirdsettling tank. The third settling tank incorporates a typical sandfilter for removing or catching particles before the water is channeledinto a storage tank. The storage tank is arranged to permit a bacteriakilling source to be discharged into the stored water.

Another sewage treatment system is the so-called CANWEL system (a trademark of Central Mortgage and Housing Corporation, Canada). The systemconsists of an absorption bio-oxidation (A-B) reactor, a sludgeseparator, a chemical reactor-clarifier, an ozone reactor, and anoptional sludge thickener, depending upon the method proposed for sludgedisposal.

The raw or comminuted domestic or general municipal (non-industrial)sewage enters the A-B reactor where, in the aeration chamber-air lift,it is contacted with activated sludge and distributed into the toplayers of the reactor mixed liquor. The activated sludge consists of amixed microbial population together with powdered activated carbon.

The sludge is kept in a downward oriented motion, maintained bywithdrawing sludge from the bottom of the reactor and recycling it tothe top via airlift. The organic matter present in sewage is primarilyremoved from the processed wastewater by adsorption on biological solidsand activated carbon. Then, as the sludge solids move downward bygravity and by the downward flow of the mixed liquor, the adsorbedorganics are bio-oxidized. Along the bio-oxidation of organics, thenitrifying bacteria present in mixed liquor suspended solids transformthe ammonia to nitrite and nitrate. As a result of these bio-reactions,the concentration of the dissolved oxygen in the reactor liquor dropsfrom about 1.5 mg/l at the top layers to near zero at the bottom layers.At this low level of dissolved oxygen, nitrite and nitrate replaceoxygen as the final electron acceptor in the respiration chain,resulting in microbial denitrification. The mixed liquor then overflowsvia a submerged pipe into the sludge separator.

The mixed liquor withdrawn from the bottom of the A-B reactor overflowsinto the sludge separator where the solids are separator from the liquorby settling, and recycled back into the reactor.

The A-B reactor system is consistently "filled" with sludge, the yield,or excess, of which automatically overflows with the treated waste-waterfrom the sludge separator into the chemical reactor-clarifier. Here, bythe addition of a chemical coagulant, phosphorus and colloidalsubstances are precipitated, and then removed together with excessbiological solids in a fluidized bed of chemical sludge.

The clarified effluent from the chemical reactor-clarifier is air-liftedinto the central column of the ozone reactor where it is contacted withozone in countercurrent fashion. Oxidation of residual organics thentakes place in the main tank. At this point the effluent is suitable forfinal disposal and may be considered for undiluted surface discharge,discharge to storm systems or small streams and lakes, or for utilityuses.

The excess chemical sludge together with the excess biological sludgecontinuously overflows into a sludge thickener, from where the thickenedsludge is periodically withdrawn and disposed of by conventional orother means. The supernatant is returned to the A-B reactor.

The CANWEL system also includes a water polishing sub-system whichconsists of mixed media filtration, optional pH balance control,optional reverse osmosis (R.O.) and disinfection with ozone. The mixedmedia gravity filtration unit accepts raw water from an approved sourceand of a quality with potential for municipal and domestic use, andremoves a high percentage of suspended solids. Where required,appropriate chemicals are added automatically in controlled amounts tomaintain pH balance.

Where required, the R.O. process demineralizes the water by reducing tosafe levels the incidence of dissolved salts. The process also dealseffectively with colloidal solids (e.g., asbestos), bacteria, anddissolved organic materials. A concentrated brine with other rejects isaccumulated for disposal.

The water flows into the ozone contact chamber for oxidation anddestruction of any residual viral and bacterial organisms. This polishedwater is now suitable for all domestic purposes and will meet therequirements for municipal water.

(iii) Deficiencies of the Prior Art

In spite of the above described prior art, there is still a need for animproved sewage treatment system, method and apparatus to provide aneffluent that meets the standards for human consumption out of sewage,in the shortest possible time. The effluent should be so pure that therewould be no need for the cleaned effluent to pass through conventionalfirst and second filtering tanks.

SUMMARY OF THE INVENTION

(i) Aims of the Invention

Accordingly one broad object of this invention is the provision of animproved sewage treating which is simple and effective in providingpotable water from raw sewage.

Another object of this invention is to provide an improved sewagetreating process which is simple and effective in providing potablewater from new sewage.

Yet another object of this invention is to provide an improved sewageseparator chamber which is adapted to be used in the sewage treatingsystem.

Yet another object of this invention is to provide such an improvedsewage separator which is simple in construction and economical inmanufacture.

(ii) Statements of Invention

This invention provides a sewage purification system comprising (a) apump for drawing sewage into the system (b) an injection inlet forinjecting a first chemical into the effluent line from the pump; (c) afirst U-shaped mixing chamber, and a second, interconnected, U-shapedmixing chamber, each of the first and second mixing chambers comprisinga near arm and a far arm, the first and second mixing chambers beingconnected to the effluent line from the pump, the inlet to the mixingchambers being to a near arm of a first mixing chamber, the outlet fromthe mixing chambers being from a far arm of a second mixing chamber, themixing chambers being interconnected between the far arm of the firstmixing chamber and the near arm of the second mixing chamber, each ofthe near arm and the far arm of the mixing chamber having bafflestherein, the baffles extending only partly across the cross-section ofthe arm and alternating from one longitudinal face to the opposedlongitudinal face and each being inclined at the same angle towards thedirection of flow, to tend to cause turbulent flow, and a conduitinterconnecting each near arm with each far arm, each conduit includingan injection inlet for the injection thereinto of a respective secondchemical and third chemical into the respective mixing chambers; (d) asplitter chamber connected to the common outlet from the mixing chambersto convert the single inflow into a pair of parallel flow outletstherefrom; (e) a separator chamber connected to the parallel flowoutlets and having inlet structure to cause streams of thechemically-treated sewage to flow in a unitary direction longitudinallyalong the inside walls of the separator chamber, the separator chamberalso having structure therein for separating sludge therefrom andwithdrawing it from the bottom thereof through a sludge withdrawal line,and for withdrawing clean water from the upper central core thereof; and(f) a sludge recycle line interconnecting the separator chamber and thepump inlet line.

(iii) Other Features of the Invention

By one feature of the sewage purification system embodiment of thisinvention, the system includes a sludge holding hopper at the base ofthe separator chamber.

By another feature of the sewage purification system embodiment of thisinvention, the system includes a sludge sweeping member at the base ofthe separator chamber.

By yet another feature of the sewage purification system embodiment ofthis invention, the system includes a sludge withdrawal line from theholding hopper.

By still another feature of the sewage purification system embodiment ofthis invention, the system includes a valve to control the recycle flowof sludge from the separator to the pump.

By another feature of the sewage purification system embodiment of thisinvention, the system includes a valved connector line between thesewage inlet line and the pump effluent line to provide for recycle fromthe pump effluent line back to the pump.

By one feature of this invention, the inlets comprises bulbous side wallprotrussions.

By another feature of this invention, the inlets feed a directingchamber defined by a double open ended cylinder having dependingcylindrical walls and a protruding annular rim by which it is secured tothe inside of the separator chamber.

By a further feature of this invention, the directing chamber provides acontinuous downwardly directed outlet door, and a central upwardlydirected part.

(iv) Generalized Description of the Invention

The raw sewage, according to one embodiment to be described herein, ispretreated to induce coagulation, by the sequential use of threedifferent chemical agents, of solids therein, both before entering into,and while within, a mixing zone. These certain chemicals aresequentially mixed with the liquid, preferably through a plurality ofinjector nozzles. The chemically treated liquid, e.g., sewage, theneventually flows to the separator chamber in a manner to be describedhereinafter.

In the particular embodiment to be described herein, the sewage to beclarified is first treated by being mixed sequentially with threedifferent chemicals, which are injected into the mixing chamber. It isimperative that the chemicals and the quantities be used in propersequence in order to effect the desired results. The chemicals used, thequantities and the sequence will depend upon design parameters and willbe described in greater detail hereinafter. They co-act to form flocs,crystals and other solids from the dissolved constituents of the sewagebeing treated, and it is these solids which are to be removed by theseparator chamber.

Once treated, the chemical reactions commence and the sludge isconverted to a coagulated waste. As the treated effluent passes throughthe separator chamber, the reacted sludge settles and is removed in themanner to be described in greater detail hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings,

FIG. 1 is a schematic representation of the system, and method of sewagetreatment of an embodiment of this invention; and

FIG. 2 is a section through the line II--II of a separator chamberaccording to another embodiment of this invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

(i) Description of FIG. 1

The sewage treatment system 10 includes a raw sewage inflow line 11leading to a main flow line 12 connected to the inflow of a pump 13. Theoutflow line 14 from the pump 13 is provided with an injection means 15for the introduction of a precipitation chemical, e.g. alum thereinto.

As used herein, alum means a hydrated double sulfate of aluminum andunivalent metals, e.g. K, Na, NH₄ ; for example, K₂ SO₄.Al₂ (SO₄)₃.24H₂O; (NH₄)₂.Al₂ (SO₄)₃.24H₂ O; KAl(SO₄)₂.12H₂ O; and (NH₄)₂ SO₄.Fe₂(SO₄)₃.24H₂ O. The alum-treated raw sewage enters at one inlet 16 of amixing chamber 17 and exits at an outlet 18 to be conveyed via line 19to the inlet 20 of a splitter chamber 21.

Mixing chamber 17 comprises two interconnected U-shaped chambers, namelya first U-shaped chamber 171 and a second U-shaped chamber 172. Theinlet 16 is connected to the near arm 173 of the first U-shaped chamber171, and the outlet 18 is connected to the far arm 176 of the secondU-shaped chamber 172. The first U-shaped chamber 171 is connected to thesecond U-shaped chamber 172 by a connecting arm 176.

The near arm 173 of the first U-shaped chamber 171 is connected to thefar arm 186 of the first U-shaped chamber 171 by means of a connectingconduit 177, which is provided with an injection means 178 for theinjection of treating chemicals thereinto. Similarly the near arm 175 ofthe second U-shaped chamber 172 is connected to the far arm 176 of theU-shaped chamber 172 by means of a connecting conduit 179, which is inturn provided with an injection means 180 for the injection of treatingchemicals thereinto. Each of the four arms 173, 174, 175, 176 isprovided with a plurality of staggered baffles 181 inclined towards thedirection of flow, to cause the chemically-treated sewage to follow asinusoidal flow path and thus be thoroughly mixed.

The splitter chamber 21 is a generally cylindrical vessel including aninternal diamond-shaped baffle 22 therein to split thechemically-treated sewage flow into two turbulent streams, separated bya vertical imperforate baffle wall 23, to flow out of first outlet 24 tofirst effluent line 25 and second outlet 26 to second effluent line 27.First effluent line 25 feed the chemically-treated sewage to first inlet29 of a cylindrical separator chamber 30, while second effluent line 27feed the chemically-treated sewage to a second, diametrically-opposedinlet 28 of the separator chamber 30.

(ii) Description of FIG. 2

Separator chamber 30 includes an annular intermediate, bulbous-walleddirecting chamber 31 which is fed by effluent lines 25, 27 and which isdefined by a double open ended cylinder 32 having depending cylindricalwalls 33 and a securing annular rim 331, by which cylinder 32 is securedto the inner wall 332. Cylinder 32 provides a continuous annulardownwardly directed outlet port 34, and a central open upwardly directedport 35. Port 34 cause the chemically-treated sewage to flow downwardlyin an annular flow adjacent the cylindrical side walls of thecylindrical chamber 30.

The chamber 30 is divided into an upper clean water holding chamber 36and a lower precipitation chamber 37. Clean water flow, upwardly throughcentral core of the chamber 30 passes through core port 35 into theupper clear water holding chamber 36, from whence it is withdrawn viacentral, valved outlet line 38.

The precipitation sludge falls to the bottom of precipitation zone 37from whence it is scraped by sweep 39 rotatably driven by rotor 40 intoa sludge hopper 42. The sludge is propelled from the sludge holdinghopper 42 by auger 41 driven by motor 43 into a waste sludge line 44,and into a recycle line 44, valved at 441, leading to main flow line 12.Main flow line 12 is connected to outflow line 14 by pump recycle line45, valved at 451.

OPERATION OF PREFERRED EMBODIMENTS

In use, the basic flow pattern is as follows. The raw sewage is drawn bypump 13 through lines 11 and 12 and is pumped to the mixing chamber 17.Before the raw sewage goes to the mixing chamber 17 it is mixed withrecycle sludge in line 4, and a recycle mixing flow both to the pump 13occurs through recycle line 45. The sludge recycle rate is about 1-20%,preferably about 5-7% of the total flow rate through the system, whichvaries from about 5-40 gallon/minute. Alum is added to the pump outflowline 14, in an amount of about 10-200, preferably about 20-50 ppm byweight.

In the mixing chambers 17, the chemicals are added to provide thechemically treated raw sewage. The chemicals are added, preferablycontinuously, but in the essential sequence as follows: First, addedthrough nozzle 178 is the second essential chemical added, namely, ahigh cationic polyelectrolyte, preferably an acrylamide-based copolymer,typically that known by the Trade Marks HERCULOC 855, HERCULOC 849 orHERCULOC 848. Other polyelectrolytes which may be used with varyingdegrees of success include high molecular weight electrolytes of eithernatural origin (e.g. proteins or polysaccharides) or of synthetic origin(e.g. alkyl halide addition products of polyvinyl pyridime).

Next the third essential chemical added, namely an anionic surfaceactive agent, preferably, a polyacrylamide base, e.g. that known by theTrade Mark DOW A 23. Other such polyacrylamide bases which may be usedare those known by the Trade Marks DOW A 23P, DOW A 25 and DOW A 27.

The particular construction of the mixing chambers 17 causes intimatetreatment of the raw sewage with all the chemicals.

An important variant in the method is that there is a recycle of pumpeffluent back to the pump through valve 45. The pump recycle rate isabout 1-20%, preferably about 5-7% of the total flow rate through thesystem. The intimately mixed chemically treated sewage passes to thesplitter chamber 21 and the spit flows go to the separation chamber 30.The sludge settles at the bottom and goes to sludge disposal via line41, or in amounts of about 1-20%, to the sludge recycle line 44, whilethe clean potable water is withdrawn via outlet line 38.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions. Consequently, such changes and modifications are properly,equitably, and "intended" to be, within the full range of equivalence ofthe following claims.

I claim:
 1. A sewage purification system comprising:(a) a pump fordrawing sewage into said system; (b) an injection inlet for injecting afirst chemical into the effluent line from said pump; (c) a firstU-shaped mixing chamber and a second interconnecting U-shaped mixingchamber, said first and second mixing chambers being connected to saideffluent line from said pump, each of said first mixing chamber and saidsecond mixing chamber comprising a near arm and a far arm, the inlet tosaid mixing chambers being to a near arm of a first said mixing chamber,the outlet from said mixing chambers being from a far arm of a secondsaid mixing chamber, the mixing chambers being interconnected betweenthe far arm of said first mixing chamber and the near arm of said secondmixing chamber, each of said near arm and said far arm having bafflestherein, said baffles extending only partly across the cross-section ofsaid arm and alternating from one longitudinal face to the opposedlongitudinal face and each being inclined at the same angle, towards thedirection of flow, to tend to cause turbulent flow, and a conduitinterconnecting each said near arm with each said far arm, each saidconduit including an injection inlet for the injection thereinto of arespective second chemical and respective third chemical into saidrespective mixing chambers; (d) a splitter chamber connected to thecommon outlet from said mixing chambers to convert said single flow intoa pair of parallel flow outlets therefrom; (e) a separator chamberconnected to said parallel flow outlets and having inlet structuretherein to cause streams of said chemicallytreated sewage to flow in aunitary direction tengentially along the side walls of said separatorchamber, said separator chamber also including means therein forseparating sludge therefrom and withdrawing it from the bottom thereof,through a sludge withdrawal line and for withdrawing clean water fromthe upper central core thereof; and (f) a sludge recycle lineinterconnecting said separator chamber and said pump inlet line.
 2. Thesewage purification system of claim 1 including a sludge holding hopperat the base of said separator chamber.
 3. The sewage purification systemof claim 2 including a sludge withdrawal line from said holding hopper.4. The sewage purification system of claim 2 wherein said sludge recycleline interconnects said sludge holding hopper to said pump inlet line.5. The sewage purification system of claim 1 including a sludge sweepingmember at the base of said separator chamber.
 6. The sewage purificationsystem of claim 1 including a valve to control the recycle flow ofsludge from said separator to said pump.
 7. The sewage purificationsystem of claim 1 including a valved connector line between the sewageinlet line and the pump effluent line to provide for recycle from pumpeffluent line back to said pump.
 8. The sewage purification system ofclaim 1 wherein said inlet structure comprises a pair of diametricallyopposed inlets adjacent the upper regions of said separator chamber,said inlets each including a protrussion from the associated side wallof said separator chamber leading to an associated inner directingchamber having a downwardly directed outlet, thereby to direct outwardflow annularly downwardly within said separator chamber.
 9. The sewagepurification system of claim 8 wherein sid inlets each comprise bulbousside wall protrusions.
 10. The sewage purification system of claim 8wherein said inlets feed a directing chamber defined by a cylinder, openended at both ends, having depending cylindrical walls and a protrudingannular rim by which it is secured to the inside of said separatorchamber.
 11. The sewage purification system of claim 10 wherein saiddirecting chamber provides a continuous, downwardly-directed, outletdoor, and a central, upwardly-directed port.
 12. The sewage purificationsystem of claim 8 wherein said means for withdrawing clean water fromthe upper central core of said separator chamber comprises a valvedcentral outlet.
 13. The sewage purification system of claim 1 whereinsaid means for separating sludge and withdrawing said sludge from thebottom of said separator chamber comprises:a holding hopper at the baseof said separator chamber; a lower sweep mechanism to force sludge to asaid hopper; means for removing waste sludge from said hopper; and meansfor withdrawing a predetermined amount of sludge from said hopper forrecycling said withdrawn sludge.
 14. The sewage purification system ofclaim 13 wherein said sludge recycle and sludge withdrawal is providedby an auger within said sludge holding hopper.